Before a tumor patient is irradiated, the calculated dose distribution of the treatment plan has to be checked. Since the measurements cannot be performed in the patient himself, phantoms are used. The dose distribution can be checked by recalculating the treatment plan for a “verification phantom”. The measured dose distribution of one or various randomly selected layers in the verification phantom is compared with the calculated dose distribution (plan related verification). Alternatively, the dose distribution of each single field is calculated and measured (field related verification). The advantages and disadvantages of these verification methods have been discussed in detail in the medical physics literature. In the current practice of radio oncology clinics, the dose distribution is checked only randomly and in two dimensions (2D).
The 2D dose distribution can be measured with a variety of devices such as films or electronic 2D arrays and EPIDs (Electronic Portal Imaging Device). The latter equipments show the advantage to make available the measured values in an electronic form. Several companies offer electronic 2D arrays and appropriate software, to verify the radiation dose distribution in a plane of special interest. The company Varian introduced a practical tool on the market, which is routinely used in many institutions for field-related 2D verification. 3D verification methods allow checking the integral dose distribution within a volume of interest. Relevant structures (body contour, anatomical structures, the planning target volume, PTV, and critical organs) and the dose distribution can be superimposed on the 3D dose distribution. By this means, evaluations and statements regarding the integral agreement between measurements and calculations are achievable, e.g., presented as a structure-based “Gamma-volume” diagram [Low et al., 1998, A technique for the quantitative evaluation of dose distributions, full cite, the content of which is incorporated herein by reference thereto]. Discrepancies between measurement and calculation can be localized and scored relative to the patient contours. Deviations outside the body contour are for instance not relevant.
Different methods have been developed to measure the 3D dose distribution in a phantom (gel dosimetry, a spirally wounded film in a phantom and mathematical estimation of doses in points outside the measurement plane. These methods are quite elaborate and time-consuming and therefore not suitable for clinical routine: First of all, the 2D measurement signal has to be converted digitally (e.g., film scanning). To convert the measured values into a radiation dose, a conversion function has to be determined, etc. Various groups succeeded in determining the 3D dose distribution, based on EPID measurements. As this equipment is attached to the linear accelerator and has a high spatial resolution, it offers significant advantages compared with other equipment. The industry has not adopted and implemented until now the 3D dose verification with EPIDs.
New products are currently on the market which determine the 3D dose distribution in a phantom based on 2D measurements. As an example, the verification phantoms “Delta4” (available from Scandidos Inc.) and “ArcCheck” (Sun Nuclear Inc.) are presented here.
The cylindrical instrument “Delta4” (IBA) incorporates two 2D detectors, which intersect in the phantom symmetry axis by 90°. The dose distribution outside the measurement planes is calculated. The beam impinges on the detectors partly at small angles, influencing the sensitivity in an adverse way, and limiting the number of detector elements suitable for measurements. (FIG. 1A). The detectors of the cylindrical “ArcCheck” (Sun Nuclear) phantom are located regularly on a cylinder shape below the phantom surface. Detectors next to the phantom symmetry axis (seen from the beam focus) are irradiated perpendicularly and show good measurement results. However, when the angle between the incident beam and the phantom symmetry axis is increased, the incident angle of the beam to the detectors decreases. Therefore, detectors at the peripheries of the phantom are only suitable for measurements when the sensitivity is angle corrected (FIG. 1B), which introduces an additional measurement error. A considerable percentage of the detectors thus cannot be used for the measurement without restrictions at a given gantry angle. The dose within the curved, closed measurement surface is set in relation to the measured dose at the detector positions. This gives information how to calculate the “measured” dose derived from the calculated one.
The Compass measurement system (IBA dosimetry): The 2D array detector is attached to the gantry and measures the field fluence (not the radiation dose). Since the measurement is performed distant to the isocenter plane, complex calculation algorithms are required to calculate the dose in a virtual 3D phantom.
As shown in FIG. 1A, the beam path is perpendicular to the detector plane 1. It delivers a well-defined measurement signal. For the detector plane 2, the opposite is true. In FIG. 1B, the beam path c is perpendicular to the detector plane on the entrance and exit side of the beam and therefore a well measurement signal is expected. For the beam path b, the opposite is true.
Due to the phantom construction of the presented 3D verification phantoms and the limited accuracy to place them in relation to the beam coordinate system, the 3D phantoms actually in use are not suitable to perform fast and accurate machine checks, especially for MLC checks (multi leaf collimator). What is needed therefore is a phantom construction that is reproducibly related to the beam coordinate system with its accessory holder. Further, what is needed is a QC accessory that is more universally applicable.
Towards this end, Häring et al presented a particular member of the RSC Phantom class. (See Häring et al., “Hochauflösende 3D IMRT Verifikation mit einem Ionisationskammerarray”, Proceedings of the 39th Annual Meeting of the DGMP, Oldenburg, p. 222-223, 2008, the content of which is incorporated by reference in.). Häring et al used a semi-cylindrical phantom body. Nevertheless, Häring et al's efforts highlighted problems in calculating the dose distribution, particularly, next to the phantom surface. Cylindrical phantom shapes are therefore not suitable to verify techniques with varying collimator or table angles, when they are fixed to the gantry. What is needed therefore is a phantom class that is suitable for such purpose.